1. Field of Invention
The present invention relates to a pixel structure and an active matrix substrate and, more particularly, to a pixel structure and an active matrix substrate capable of avoiding image sticking phenomenon.
2. Description of the Related Art
Nowadays, it is the trend that a TFT-LCD (thin film transistor liquid crystal display) should have the characteristics such as high contrast ratio, no gray scale inversion, little color shift, high luminance, high color richness, high color saturation, fast response and wide viewing angle. Among the wide viewing technologies, there are a few types of LCDs, such as TN+FILM (twisted nematic LCD plus wide viewing film), IPS-LCD (in-plane switching LCD), FFS-LCD (fringe field switching LCD) and MVA TFT-LCD (multi-domain vertical alignment thin film transistor-LCD).
FIG. 1 is a schematic section drawing of a conventional MVA TFT-LCD panel. Referring to FIG. 1, a conventional MVA TFT-LCD panel 100 includes a TFT array substrate 110, a color filter substrate 120 and a liquid crystal layer 130 located between the TFT array substrate 110 and the color filter substrate 120. The TFT array substrate 110 has a plurality of pixel electrodes 112 with slits 112a. The color filter substrate 120 includes a common electrode 122 and a plurality of alignment protrusions 124 disposed on the common electrode 122. Since the liquid crystal molecules of the LC layer 130 can be arranged in multi-directions by the alignment protrusions 124 disposed on the common electrode 122 and the slits 112a formed in the pixel electrodes 112, a plurality of different domains A and B are formed. In this way, the requirement of wide viewing angle can be obtained by the MVA TFT-LCD panel 100. Remarkably, within a single pixel structure, the LC capacitances (liquid crystal capacitance) Clc in the domain A and the domain B are the same. On the other hand, as a viewer sees the MVA TFT-LCD panel 100 from different angles, the corresponded transmittance-gray level curves are varied with the angles. In other words, when a viewer sees from a different angle, the brightness (gray level) displayed by the conventional MVA TFT-LCD panel 100 varies, which results in color washout phenomenon.
FIGS. 2A and 2B are schematic section drawings of another conventional MVA TFT-LCD panel. Referring to FIGS. 2A and 2B, to avoid the above-mentioned color washout phenomenon, another conventional MVA TFT-LCD panel 200 has been developed, which includes a TFT array substrate 210, a color filter substrate 220 and a liquid crystal layer 230 located between the TFT array substrate 210 and the color filter substrate 220. The TFT array substrate 210 has a plurality of pixel electrodes 212 and floating pixel electrodes 214. The pixel electrodes 212 and the floating pixel electrodes 214 have slits (not shown in the figures). The color filter substrate 220 includes a common electrode 222 and a plurality of alignment protrusions 224 disposed on the common electrode 222.
Note that the pixel electrodes 212 and the floating pixel electrodes 214 are electrically isolated from each other in every single pixel structure. When the MVA TFT-LCD panel 200 is driven to display images, the pixel electrode 212 is coupled to a voltage level Vdata, and the floating pixel electrode 214 keeps a voltage level Vcc by capacitor coupling effect (C.C.), wherein Vdata>Vcc. The MVA TFT-LCD panel 200 is accordingly divided into a domain A, a domain B, a domain A′ and a domain B′. Thus, in a single pixel structure, the pixel electrode 212 and the floating pixel electrode 214 have different voltage levels for compensating color saturation while the pixel structure is driven by the same signal. The LC capacitance over the pixel electrode 212 keeps Clc1 (corresponding to the domains A and B) herein, while the LC capacitance over the pixel electrode 214 keeps Clc2 (corresponding to the domains A′ and B′), and Clc1>Clc2.
As shown in FIG. 2B, since the floating pixel electrode 214 is electrically floated, during the MVA TFT-LCD panel 200 is being driven, the charges are easily trapped in an alignment film PI if an unbalance between positive charges and negative charges is present. Thus, the voltage level of the floating pixel electrode 214 is accordingly shifted, which causes a serious image sticking phenomenon in the above-described wide viewing technologies.